The present invention relates to periodic signal detecting circuits and more particularly to a circuit employing a comparator circuit for comparing an input periodic signal waveform with a predetermined threshold voltage and generating an output signal, e.g., a circuit for reshaping the waveform of a periodic signal such as a signal from a rotational position detecting electromagnetic pickup coil and generating an output signal or a circuit for detecting the rotation of a crankshaft of an internal combustion engine.
The known circuits of the above type are constructed so that, as shown in FIG. 1, a threshold voltage is generated by a feedback resistor or CR time constant circuit connected between a comparator output terminal and a comparison input terminal and a grounded terminal and the threshold voltage is compared with an input signal waveform from an electromagnetic pickup coil thereby generating an output signal. Where the feedback resistor is simply connected, there is a disadvantage that the circuit tends to detect the noise superposed on an input waveform due to the threshold voltage being constant. Also the circuit in which the CR time constant circuit is connected is disadvantageous in that if the time constant is large and the output signal changes its state in response to the positive-going portion of the input signal, the output signal is not necessarily reset in response to the zero crossing of the negative-going portion of the input signal and the detection of rotational positions becomes inaccurate. If the time constant is small, while the circuit does not detect noise just after the change of state of the output signal, it tends to detect noise after a period determined by the time constant CR. Also, in order to prevent any erroneous operation due to noise superposed on an input waveform and in order to vary the triggering level in response to the input signal frequency so as to prevent any variation of the triggering point angular position (e.g., variation of the crankshaft position in the case of an internal combustion engine ignition system) due to variation of the input signal frequency, a circuit construction has been proposed in which a plurality of time constant circuits are included to provide different threshold voltages (varying in the same polarity with the input signal waveform) and the threshold voltages are selectively coupled to a comparator circuit. This construction has the disadvantage of being complicated in circuit construction.